This invention relates to the measurement of gas characteristics through the use of a probe comprising a solid electrolyte which is in contact on one side or face with a reference gas such as the ambient atmosphere and is in contact on the opposing side or face with a hot gas whose characteristics are to be measured. In such a probe, a voltage is generated between the two sides or faces of the solid electrolyte, the magnitude of the voltage being dependent upon the temperature of the electrolyte and on the ratio of the oxygen partial pressure on opposing sides or faces of the electrolyte. This principle has been used in the past to measure the oxygen partial pressure of hot furnace gases with various different oxygen sensors, such as disclosed in U.S. Pat. Nos. 3,454,486; 3,546,086 and 3,597,345 and British Pat. No. 1,296,995.
Although the principle of operation of the above-noted type of oxygen sensors is quite simple, the utilization of such oxygen sensors in industrial applications has been limited in the past due to several practical problems. First, the solid electrolyte, which is a ceramic material, is hard and brittle and is thus very susceptible to thermal shock and/or mechanical damage. Another problem has been the detachment of the electrode from the surface of the electrolyte due to corrosion of the electrodes. The latter problem occurs even when the electrodes are made of a noble metal, such as platinum. For example, platinum reacts at high temperatures and low oxygen pressures with stabilized zirconia electrolyte material, which is commonly used in this type of oxygen sensor to form ZrPt.sub.3. The reaction product is in the form of fragments or dust which drops away from the electrode, thus reducing the thickness thereof and eventually breaking the electrical contact between the electrode and the electrolyte or loosening the electrode sufficiently in its mounting to allow it to be blown away by the gas stream under measurement. The life of prior art electrodes is substantially reduced when used in a highly corrosive atmosphere typically present in some heat treating furnaces. For the above reasons, the useful working life of the above-noted type of oxygen sensor has been limited in the past and has limited their potential range of application.
The use of an electronic ceramic coating is disclosed in U.S. Pat. No. Re. 28,792. This patent discloses (Col. 6, lines 19-66) an electrode for a fuel cell using an electronic conducting porous metal oxide coating. However, the type of electrode disclosed in U.S. Pat. No. Re. 28,792 requires that the electrodes 12 and 14 which are applied directly to the electrolyte be porous so that gas can readily diffuse through the electrodes to the electrolyte interface 10 (U.S. Pat. No. Re. 28,792, Col. 3, lines 48-68; Col. 4, lines 1-8). This arrangement requires a somewhat critical and low contact resistance (Col. 6, lines 19-20) for effective operation. Inasmuch as the present invention is an open circuit sensor, the contact resistance is not as critical for good operation. It is difficult to obtain uniform porosity with coatings. If the coating is too thick and does not have sufficient porosity, electrode polarization can occur which will affect output of the cell. If the coating is too thin, the resistance may be too high for proper operation. The disadvantages and critical requirements of electronic conducting coatings applied directly to the electrolyte are discussed in the article "Cathode Materials and Performance in High-Temperature Zirconia Electrolyte Fuel Cells" in the Journal of the Electrochemical Society, Vol. 116, pages 1170-1175 (1969). Use of a disc or other element formed from electronic conducting ceramic as an electrode in accordance with the invention eliminates the need for porosity in a coating applied to an electrolyte as taught by the prior art. The mechanical clearance between the electronic conducting ceramic electrode and the electrolyte enables accessibility of the furnace gases.